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a bit more complex - storm trenches

a bit more complex - storm trenches

a bit more complex - storm trenches

A model I am working on now is a storm system designed to dissipate the first .5" of rain from a parking lot into the subsurface, and to relay any further runoff through a pipe network to the city's storm sewer.  The catch basins have two outlets - the first (lower elevation) is a 6" perforated pipe that runs through crushed stone to dissipate the first half inch of rain (storm trench).  The second outlet is 2' above the perforated pipe and is a 15" storm pipe that conveys overflow to the city's system.

My question is how to model this system.  Currently, the catch basin is a pond, with a primary (storm trench) and secondary (15" storm sewer) outlets.  The storm trench is a pond.  The outlet to this is an exfilter (discarded).  Right now too much water flows through the trench, the water won't backup into the catch basin and flow out of the 15" overflow pipe.  What is a better way of modeling my storm trench?  This has been difficult to explain in words, please let me know where I need to elaborate more.  Thanks, hydrocad experts...

RE: a bit more complex - storm trenches

A drainage area of 2 acres of paved parking lot will generate about 1.2 cfs for 0.50" 24 hour rainfall.  A 6" orifice with a 2' head will discharge 1.25 cfs.  CHeck the inflow.  If it is less than 1.25 cfs, it will all go out the 6" pipe.  Set the invert differential between the 6" and the 15" to equal the head necessary to discharge only the 0.50" rainfall.  Any storm in excess of 0.50" will then go out the 15" pipe.

If you'd like, send me an e-mail with the attachement and I will review it for you.


RE: a bit more complex - storm trenches

Thanks for the response.  The scenario that you described is occuring in my model.  

What is your opinion on the storm trench?  Is the exfilter correct to model the drainage?

Look forward to your insight.


RE: a bit more complex - storm trenches

Better than an exfiltrator trench would be to use a Bioretention area (Rain Garden)  This would not only infiltrate the runoff, but provide some treatment as well.  If you go to Yahoo or Google and search for bioretention you will get numerous sites describing this method.  The bioretention area can be incorporated in the landscape of the site.  The discharge would be above ground into the Rain Garden where it will be stored and percolate through the mulch and soil either into the adjacent soil (if permeable) or into an underdrain system (if clay soil).  This method removes pollutants and cools the "First Flush" before it enters the receiving body (stream, lake, etc.)  The rain garden also has an overflow to take care of the excess runoff from larger storms.  (this runoff is cleaner than the first flush or 0.50" runoff.)

RE: a bit more complex - storm trenches

Model the Perf pipe and crushed stone as a reservoir.  Volume in the pipe + 33% voids in the stone.  This off line water quality/quantity device will be prone to clogging without adequate pre-treatment to remove sediment, trash etc.

Are you including soil percolation?

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